1. Field of the Invention
Exemplary aspects of the present invention generally relate to a sheet conveyance unit that separates sheets of paper placed on a sheet tray one by one to feed each sheet separately from the sheet tray, an image reading device including the sheet conveyance unit, and an image forming apparatus including the image reading device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction devices having two or more of copying, printing, and facsimile functions, typically form a toner image on a recording medium (e.g., a sheet of paper, etc.) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image bearing member (e.g., a photoconductor); an irradiating device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device develops the electrostatic latent image with a developer (e.g., toner) to form a toner image on the photoconductor; a transfer device transfers the toner image formed on the photoconductor onto a sheet of recording media; and a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
An image reading device such as a scanner used for the image forming apparatus often employs a sheet-through method, in which an image on a sheet of an original document is read by an image reading unit fixed to the image reading device while the document is conveyed past the device at a predetermined speed. Because the image on the document is read while the document is being conveyed, the image reading device employing the sheet-through method can achieve higher productivity compared, for example, to a book-type image reading device, in which conveyance of the document is temporarily stopped while irradiating the document with light to read the image on the document.
The image reading device employing the sheet-through method includes an automatic document conveyance unit that conveys sheets of documents one by one from a document stand, on which the documents are placed, to an image reading position, at which the image reading unit reads images on the documents. During continuous reading of the images on the documents, the automatic document conveyance unit is required to prevent multiple feeding of documents to the image reading position.
Known image reading devices include a document conveyance unit that conveys the document to the image reading position and a pickup roller that picks up a topmost document from a stack of multiple documents placed on the document stand to feed the topmost document to the document conveyance unit. The pickup roller contacts an upper surface of the topmost document on the document stand at a predetermined position in a direction of conveyance of the documents and is rotatively driven to feed the topmost document to the document conveyance unit. The image reading device further includes a separation unit that separates the topmost document fed by the pickup roller from the rest of the documents so that only the topmost document is conveyed to the document conveyance unit. As a result, multiple feeding of the documents can be prevented.
However, when multiple documents closely contacting each other reach the separation unit together at the same time, those documents cannot be separated from each other by the separation unit, causing multiple feeding of the documents to the document conveyance unit. To solve this problem, a configuration that includes a pre-separation unit disposed between the pickup roller and the separation unit is used. The pre-separation unit includes a guide member that guides the documents to the separation unit. The guide member has a sloped surface sloping upward from upstream to downstream in the conveyance direction of the documents so that resistance against the conveyance direction of the documents is applied to the documents passing over the sloped surface of the pre-separation unit. Accordingly, only the topmost document to which a conveyance force is directly applied by the pickup roller is conveyed to the separation unit against the resistance applied by the sloped surface of the pre-separation unit. By contrast, the rest of the documents, which reach the sloped surface of the pre-separation unit together with the topmost document, are separated from the topmost document by the resistance applied by the sloped surface of the pre-separation unit.
Thus, provision of the pre-separation unit can more securely prevent multiple feeding of the documents. However, in a case in which the documents conveyed to the pre-separation unit are skewed, the skew of the documents may be increased by the pre-separation unit because one of lateral corners at a leading edge of each document in the conveyance direction, which is perpendicular to the lateral direction of the documents, is positioned upstream from the other one of lateral corners thereof and receives resistance from the sloped surface of the pre-separation unit. As a result, a torque that rotates the one of the lateral corners of the document toward the upstream of the conveyance direction around a contact position, at which the document is contacted by the pickup roller, is generated. Consequently, the skew of the document is further increased.
The increase in the skew of the documents may occur also in a case in which the surface of the pre-separation unit is not sloped upward. For example, even when the surface of the pre-separation unit is horizontally formed or sloped downward, the skew of the documents may be increased as long as an angle formed between the downward sloped surface and a guide portion provided immediately in front of the downward sloped surface or, in a case in which the guide portion is not provided, along the surface of a plane at the leading edge of the document immediately in front of the downward sloped surface (hereinafter referred to as plane surface direction) is less than 180°.
In addition, even in a configuration in which both the sloped surface and the plane surface direction are upward from a conveyance path of the documents, the skew of the documents may be increased when the angle between the sloped surface and the plane surface direction is less than 180°. This happens because even after the leading edge of the document is guided in the plane surface direction, the document tends to keep its shape along the conveyance direction while being conveyed in the plane surface direction due to its stiffness. At this time, when the angle between the plane surface direction and the sloped surface is less than 180°, the leading edge of the document conveyed in the plane surface direction hits against the sloped surface so that the resistance against the conveyance direction is applied to the document.
Thus, although the topmost document is separated from the rest of the documents by the resistance, the skew of the document is increased when the resistance acts only on the one of the lateral corners at the leading edge of each document.